(1) Prior incubation of Jurkat cells with phorbol ester or forskolin prevented tyrosine-phosphorylation of PLC-gamma1 as well s the hydrolysis of PtdIns 4,5-P2 induced by ligation of T-cell antigen receptor. This was shown to be correlated with the phosphorylation of serine 1248 of PLC- gamma1 by protein kinase C or cAMP-dependent kinase. Thus, phosphorylation of PLC-gamma1 by PKC or PKA may modulate the interaction of PLC-gamma1 with the protein tyrosine kinase or protein tyrosine phosphatase. (2) Cross-linking of Fc receptors for IgG, FcgammaRI or FcgammaRII, in the human monocytic cell line U937 is functionally coupled to a nonreceptor tyrosine kinase that phosphorylates PLC-gamma1 and thereby causes activation of PLC-gamma1. (3) The proposed Ca(II)-signaling actions of Ins 1,3,4,5-tetrakisphosphate were analyzed in cells transfected with rat brain Ins 1,4,5-trisphosphate 3-kinase. There was no evidence that high Ins 1,3,4,5-tetrakisphosphate levels promote Ca(II) mobilization. (4) cDNA corresponding to a previously uncharacterized phospholipase C was isolated, sequenced and expressed. The new PLC, PLC-beta2, was characterized. (5) The relative specificities of members of the Galphaq family of G proteins were tested. All four proteins of the Galphaq, Galphaq, Galpha11, Galpha14, and Galpha16, were found to stimulate PLC-beta1 with Galphaq and Galpha11 being most efficient. On the other hand, Galpha16 was found to most effectively activate PLC-beta2 while the others showed much less stimulation.